Fluid
Mosaic Model of the Cell Membrane: The
plasma membrane is a dynamic structure. It is composed of 2 layers
of lipid material with protein molecules interspersed through out.
The phospholipids move at a rate of 2um. /sec. while the proteins
move at a much slower rate. The phospholipids have 2 definite ends
, a hydrophilic (focusing to the outside and inside of the
membrane) and a hydrophobic one (forming the center of the
membrane). The proteins are of two types Integral (passing through
the width of the membrane (act as transport channels) and
peripheral (act as recognition sites). These peripheral proteins
contain carbohydrates to help in cell to cell recognition. These
carbohydrates are called oligosaccharides. I f they bind to
proteins they become glycoproteins, if they bind to lipids , they
become glycolipids. Cholesterol molecules are found in animal
membranes to help add support to its structure. The majority of
the phospholipids contain unsaturated fatty acids to keep it
fluid. Looking at the over all picture of the membrane the term
fluid mosaic should become evident.

Traffic of molecules across the
membrane:

1. Hydrophobic molecules (Hydrocarbons and
Oxygen) can cross with ease because they can dissolve in the lipid
bilayer.

These transport proteins work in the following
ways: provide a tunnel to allow material to flow through, bind to
the substance and physically move it, and they are specific for
the substance they move.

Diffusion and Passive
Transport:

Concentration gradient: is a regular
concentration change over a distance in a particular
direction.The net directional movement is
away from the center of concentration. Diffusion is the net
movement down the concentration gradient. Diffusion is caused by
the following natural occurrences: thermal motion ( movement
caused by the loss of heat), random molecular movement, and an
increase in entropy of the system. In a solution of many different
substances each substance diffuses down its own concentration
gradient independent of the others. Diffusion is a passive type of
transport, it requires no additional energy to make it work.
Osmosis is diffusion of water across a biological
membrane.

Solutions: A solution is a mixture of a
solute ( thing being dissolved) and a solvent (thing doing the
dissolving). Water is usually the solvent, hence the term aqueous
solution.

Types of Aqueous Solutions:

1. Hyperosmotic: solution with the
greater concentration of solute (salt water compared to fresh
water). If a cell from a salt water fish is placed in a beaker of
fresh water, the cell is said to be hyperosmotic to the
water.

2. Hypoosmotic: solution with a lesser
concentration of solute (fresh water compared to salt water). If a
cell from a fresh water fish is placed into a beaker of salt
water, the cell is said to be hypoosmotic to the
water.

3. Isosmotic: solutions of equal solute
concentrations are said to be isosmotic.

Facilitated Diffusion: Passive form of
transport caused by the help of carrier proteins specific for the
molecules they transport. Movement is always down the
concentration gradient. Polar molecules and ions are usually moved
in this fashion.

Active Transport: Movement of molecules up the
concentration gradient. The cell must expend its own energy to
move the substances.

Sodium-Potassium Pump: Antiport type molecule.
It takes 3 sodium ions out of the cell for every 2 potassium ions
it takes into the cell. Since sodium is more abundant outside the
cell and potassium inside the cell, the cell is working against
the concentration gradient. In order for the protein to work, it
must be energized by ATP. As this occurs the cytoplasm generates
an over all negative charge 2K as opposed to 3Na on the outside,
giving the extracellular fluid a positive charge. Another example
is a Proton pump that transports Hydrogen ions. These are called
electrogenic pumps, since they produce mild currents of
electricity. Chloroplasts and mitochondria use this to perform
energy creating products from sunlight and
macromolecules.

Endocytosis and Exocytosis:

Exocytosis: Movement of materials out of the
cell. These usually are trapped in vesicles that merge with the
plasma membrane and release their contents. They are usually
produced by the ER or Golgi Apparatus.

Endocytosis: Movement of materials into the
cell. This is subdivided based on the type and size of material
being transports.

1. Phagocytosis: Plasma membrane surrounding a
large solid piece of material or cell, forming a vacuole and
digesting it in conjunction with lysosomes.

2. Pinocytosis: plasma membrane forming a
vacuole surrounding liquids and taking them into the
cell.

3. Receptor mediated: special receptors
embedded in the membrane will pick up specific materials and bring
them into the cell.